1. Field of the Invention
The present invention relates to positioning a transducer over a disk in a disk file data storage system. More particularly, the present invention relates to demodulation of position information signals provided by the transducer in response to position information read from the disk.
2. Description of the Prior Art
Disk file data storage systems contain one or more magnetic disks on which data is stored in concentric tracks. A transducer writes, or magnetically encodes, the data on the track. The transducer is also capable of reading the magnetically encoded data from the tracks.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the transducer radially for track seek operations and holds the transducer directly over a track for track following operations.
A servo transducer reads position information from the disk and provides a position signal which is decoded by a position demodulator and presented in digital form to a servo control microprocessor. The servo control microprocessor essentially compares actual radial position of the transducer over the disk with desired position and commands the actuator to move in order to minimize position error.
In one type of servo system, one disk surface is dedicated to contain servo tracks which are encoded with servo position information. The servo position information in the servo tracks is condensed to evenly spaced sectors. A servo transducer flies over the servo sectors as the disk rotates and produces a sampling effect. The actual position transducer information is updated at the end of each servo sector through the use of track identification information and position error information. The track identification number is prewritten into each servo sector and serves as coarse transducer position information. The position error information is written in the servo sector and represents the distance that the servo transducer is located from the center of the track. This position error information serves as fine transducer position information.
The position error information is generally written in two fields. One is referred to as a quadrature field and the second is called a normal field. Position error information obtained from the normal field or the quadrature field is called a normal or quadrature position sample. By decoding the position samples obtained from these two fields, the off-track position of the transducer is determined relative to the center of the track. The position samples are typically decoded by integrating the analog position signal provided by the transducer which represents the position error information magnetically encoded on the disk. The integrated signal is then converted to a digital signal representing transducer position error.
In the past, the normal field and quadrature field, as encoded on the magnetic disk, were located adjacent one another. As the transducer flew over the normal field and then the quadrature field, the position samples from the normal and quadrature fields were decoded. However, radial movement of the transducer over the magnetic disk resulted in a slanted trajectory of the transducer with respect to the quadrature and normal fields on the magnetic disk. The slanted trajectory caused nonlinearities in the form of an apparent phase shift in the position samples decoded from the quadrature and normal fields. This phase shift resulted in nonlinearities in the integrated position signal. The nonlinearities also caused inefficient actuator control.